1. Field of the Invention
The present invention relates to a motor capable of reducing the leaking magnetic flux by forming a gap of a large magnetic resistance in the mutually close portions of mutually opposed first and second external magnetic poles, thereby increasing the output.
2. Related Background Art
Among the conventional compact motors, there is known a compact cylindrical stepping motor as shown in FIG. 24. In such motor, a stator coil 105 is concentrically wound on a bobbin 101, which is axially sandwiched and fixed by two stator yokes 106. The stator yokes 106 are respectively provided with stator teeth 106a, 106b which alternate along the internal periphery of the bobbin 101, while the stator yoke 106 integral with the stator teeth 106a or 106b is fixed to a case 103 to constitute a stator 102.
A flange 115 and a bearing 108 are fixed to one of the two cases 103, while another bearing 108 is fixed to the other case 103. A rotor 109 is composed of a rotor magnet 111 fixed to a rotor shaft 110, and the rotor magnet 111 and the stator yoke 106a of the stator 102 define radial gaps. The rotor shaft 110 is rotatably supported between the two bearings 108.
A configuration employing such a compact stepping motor employed for driving a lens of a camera is known, for example in the Japanese Patent Laid-open Application No. 3-180823. In such a configuration, an arc-shaped stepping motor is provided around an image taking lens, and a female screw is driven by the output shaft of the stepping motor to displace a male screw, fixed to a lens holder supporting a lens, parallel to the optical axis.
However, the above-described conventional compact stepping motor is associated with the drawback that it has a large external dimension, since the case 103, the bobbin 101, the stator coil 105, the stator yoke 106, etc. are concentrically provided outside the rotor.
Also, as shown in FIG. 25, the magnetic flux generated by energization of the stator coil 105 passes through an end face 106a1 of the stator tooth 106a and an end face 106b1 of the stator tooth 106b and does not effectively act on the rotor magnet 111, so that the motor output is not elevated.
The present applicant has proposed a motor capable of resolving such a drawback in the U.S. patent application Ser. No. 08/831,863 (European Patent Application No. 97105567.8).
In the proposed motor, a cylindrical rotor consists of a permanent magnet circumferentially divided in equal sections and alternately magnetized, and a first coil, the rotor and a second coil are provided in succession in the axial direction of the rotor. A first external magnetic pole and a first internal magnetic pole, magnetized by the first coil, are respectively opposed to the external and internal peripheries of the rotor, while a second external magnetic pole and a second internal magnetic pole, magnetized by the second coil, are respectively opposed to the external and internal peripheries of the rotor, and a rotary rotor shaft is connected to the cylindrical permanent magnet.
The motor of the above-described configuration has a high output and a reduced external dimension, but the tooth-shaped magnetic poles are difficult to produce because of the limited diameter of the internal magnetic poles. Also, it is desired to obtain a stable output without vibration from the motor of a small diameter.
For this reason, the present applicant has recently proposed a motor with the internal magnetic poles of an easily produceable shape in the U.S. patent application Ser. No. 08/994,994 (European Patent Application No. 97122508.1), and a rotor enabling easy mounting of output transmission means, such as a gear or a pulley, on the rotary shaft of a small diameter, thereby providing a stable output without vibration, in the U.S. patent application Ser. No. 09/022,474.
It has further been desired to obtain a high-performance motor of an increased output by reducing the leaking magnetic flux based on the relationship of the first and second stators.